During collection and storage, platelets develop a variety of structural and in vitro functional abnormalities, including alterations in surface membrane constituents, granule release, signal transduction, membrane phospholipid composition, energy metabolism and cytoskeletal organization. The development of this platelet storage lesion is the result of platelet activation, opsonization, hypermetabolism and senescence. It is also known that upon transfusion some of these in vitro abnormalities reverse; however, the definition of which abnormalities are readily reversible and which are irreversible is not well understood. Based on preliminary data, we hypothesize that (a) abnormalities previously thought to be irreversible may not be so, for example, P-selectin expression on the platelet surface; (b) a significant portion of both the activation-derived and the opsonization- derived platelet lesion is secondary to complement activation during collection and storage; and (c) that a portion of the metabolic abnormalities observed are also directly linked to platelet activation via mitochondrial dysfunction induced by normal activation events. We have developed an in vitro whole blood model of transfusion that allows one to separately analyze subsets of "transfused" platelets simultaneously with native platelets. In addition, we have previously investigated in detail the participation of specific complement components in the development of the somewhat analogous platelet lesion which is associated with extracorporeal circulation and now have preliminary data to suggest similar complement participation in the storage lesion. We now propose to: (a) define which aspects of the platelet storage lesion are reversible upon reintroduction of the stored platelets into the normal whole blood milieu using the transfusion model; and (b) define the role of specific complement components in the generation of the activation, opsonization and metabolic storage lesion under different conditions of collection and storage. We will use specific blocking molecules, multiparameter flow cytometric and image analysis technology, cytoskeletal and signal transduction analysis and newly applied metabolic assays. The long term goal of the project, which combines the synergistic expertise of several established investigators, is to improve the clinical results of platelet transfusion.